block includes
  include ../_util-fns

:marked
  One of the defining features of a single page application is its manipulation
  of the DOM tree. Instead of serving a whole new page every time a user
  navigates, whole sections of the DOM appear and disappear according
  to the application state. In this chapter we'll to look at how Angular
  manipulates the DOM and how we can do it ourselves in our own directives.

  单页面应用的基本特性之一，就是它要操纵DOM树。不同于以前那种用户每次浏览都重新从服务器取得整个页面的方式，
  单页面应用中，DOM中的各个区域会根据应用程序的状态而出现或消失。
  在本章中，我们将看看Angular如何操纵DOM树，以及我们该如何在自己的指令中这么做。

  In this chapter we will

  在本章中，我们将：
  - [learn what structural directives are](#definition)
  - [学习什么是结构型(structural)指令](#definition)
  - [study *ngIf*](#ngIf)
  - [研究*ngIf*](#ngIf)
  - [discover the `<template>` element](#template)
  - [`<template>`元素揭秘](#template)
  - [understand the asterisk (\*) in **ngFor*](#asterisk)
  - [理解**ngFor*中的星号(\*)](#asterisk)
  - [write our own structural directive](#unless)
  - [写我们自己的结构型指令](#unless)

  Try the <live-example></live-example>.
  
  试试<live-example></live-example>。

<a id="definition"></a>
.l-main-section
:marked
  ## What are structural directives?
  ## 什么是结构型指令？

  There are three kinds of Angular directives:

  Angular指令可分为三种：
  1. Components
  1. 组件
  1. Attribute directives
  1. 属性型指令
  1. Structural directives
  1. 结构型指令

  The *Component* is really a directive with a template.
  It's the most common of the three directives and we write lots of them as we build our application.

  *组件*其实就是一个带模板的指令。
  它是这三种指令中最常用的，我们会写大量的组件来构建应用程序。

  The [*Attribute* directive](attribute-directives.html) changes the appearance or behavior of an element.
  The built-in [NgStyle](template-syntax.html#ngStyle) directive, for example,
  can change several element styles at the same time.
  We can use it to render text bold, italic, and lime green by binding to a
  component property that requests such a sickening result.

  [*属性型*指令](attribute-directives.html)会修改元素的外观或行为。
  比如，内建指令[NgStyle](template-syntax.html#ngStyle)就能同时修改元素的好几个样式。
  通过绑定到组件的属性，我们可以把文本渲染成加粗、斜体、灰绿色这种肉麻的效果。

  A *Structural* directive changes the DOM layout by adding and removing DOM elements.
  We've seen three of the built-in structural directives in other chapters: [ngIf](template-syntax.html#ngIf),
  [ngSwitch](template-syntax.html#ngSwitch) and [ngFor](template-syntax.html#ngFor).

  *结构型*指令通过添加和删除DOM元素来改变DOM的布局。
  我们会在其它章节看到三个内建的结构型指令：[ngIf](template-syntax.html#ngIf)、
  [ngSwitch](template-syntax.html#ngSwitch)以及[ngFor](template-syntax.html#ngFor)。

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'structural-directives')(format=".")


<a id="ngIf"></a>
.l-main-section
:marked
  ## NgIf Case Study
  ## NgIf案例分析

  Let’s focus on `ngIf`. It's a great example of a structural
  directive: it takes a boolean and makes an entire chunk of DOM appear
  or disappear.

  我们重点看下`ngIf`。它是一个很好的结构型指令案例：它接受一个布尔值，并据此让一整块儿DOM树出现或消失。

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngIf')(format=".")

:marked
  The `ngIf` directive does not hide the element.
  Using browser developer tools we can see that, when the condition is true, the top
  paragraph is in the DOM and the bottom disused paragraph is completely
  absent from the DOM! In its place are empty `<script>` tags.

  `ngIf`指令并不会隐藏元素。
  使用浏览器的开发者工具就会看到：当`condition`为真的时候，只剩下了DOM顶部的段落，而底部无用的段落完全从DOM中消失了！
  在它的位置上是空白的`<script>`标签

figure.image-display
  img(src='/resources/images/devguide/structural-directives/element-not-in-dom.png' alt="element not in dom")

:marked
  ### Why *remove* rather than *hide*?
  ### 为什么*移除*而不是*隐藏*？
  We could hide the unwanted paragraph by setting its css `display` style to `none`.
  The element would remain in the DOM while invisible. Instead we removed it with `ngIf`.

  其实也可以通过把CSS样式`display`设置为`none`来隐藏掉那个不想要的段落。
  该元素仍然留在DOM中，只是看不到了。但我们却通过`ngIf`移除了它。

  The difference matters. When we hide an element,
  the component's behavior continues.
  It remains attached to its DOM element. It continues to listen to events.
  Angular keeps checking for changes that could affect data bindings.
  Whatever the component was doing it keeps doing.

  不同之处在于：当我们隐藏掉一个元素时，组件的行为还在继续 —— 它仍然附加在它所属的DOM元素上，
  它也仍在监听事件。Angular会继续检查哪些能影响数据绑定的变更。
  组件原本要做的那些事情仍在继续。

  Although invisible, the component &mdash; and all of its descendent components &mdash;
  tie up resources that might be more useful elsewhere.
  The performance and memory burden can be substantial and the user may not benefit at all.

  虽然不可见，组件及其各级子组件仍然占用着资源，而这些资源如果分配给别人可能会更有用。
  在性能和内存方面的负担相当可观，而用户却可能无法从中受益。

  On the positive side, showing the element again is very quick.
  The component's previous state is preserved and ready to display.
  The component doesn't re-initialize &mdash; an operation that could be expensive.

  当然，从积极的一面看，重新显示这个元素会非常快。
  组件以前的状态被保留着，并随时可以显示。
  组件不用重新初始化 —— 该操作可能会比较昂贵。

  `ngIf` is different.
  Setting `ngIf` to false **does** affect the component's resource consumption.
  Angular removes the element from DOM, stops change detection for the associated component,
  detaches it from DOM events (the attachments that it made) and destroys the component.
  The component can be garbage-collected (we hope) and free up memory.

  而`ngIf`不同。
  把`ngIf`设置为假**将会**影响到组件的资源消耗。
  Angular会从DOM中移除该元素，停止相关组件的变更检测，把它从DOM事件中摘掉(事件是组件造成的附加项)，并销毁组件。
  组件会被垃圾回收(希望如此)并释放内存。

  Components often have child components which themselves have children.
  All of them are destroyed when `ngIf` destroys the common ancestor.
  This cleanup effort is usually a good thing.

  组件通常还有子组件，子组件还有自己的子组件。
  当`ngIf`销毁这个祖先组件时，它们全都会被销毁。
  这种清理工作通常会是好事。

  Of course it isn't *always* a good thing.
  It might be a bad thing if we need that particular component again soon.

  当然，它也并不*总是*好事。
  如果我们很快就会再次需要这个组件，它就变成坏事了。

  The component's state might be expensive to re-construct.
  When `ngIf` becomes `true` again, Angular recreates the component and its subtree.
  Angular runs every component's initialization logic again. That could be expensive ... as when
  a component re-fetches data that had been in memory just moments ago.

  重建组件的状态可能是昂贵的。
  当`ngIf`重新变为`true`的时候，Angular会重新创建该组件及其子树。
  Angular会重新运行每个组件的初始化逻辑。那可能会很昂贵……比如当组件需要重新获取刚刚还在内存中的数据时。
.l-sub-section
  :marked
    *Design thought*: minimize initialization effort and consider caching state in a
    companion service.

    *设计思路*：要最小化初始化的成本，并考虑把状态缓存在一个伴生的服务中。
:marked
  Although there are pros and cons to each approach,
  in general it is best to use `ngIf` to remove unwanted components rather than
  hide them.

  虽然每种方法都有各自的优点和缺点，但使用`ngIf`来移除不需要的组件通常都会比隐藏它们更好一些。

  **These same considerations apply to every structural directive, whether built-in or custom.**
  We should ask ourselves &mdash; and the users of our directives &mdash; to think carefully
  about the consequences of adding and removing elements and of creating and destroying components.

  **同样的考量也适用于每一个结构型指令，无论是内建的还是自定义的。**
  我们应该提醒自己以及我们指令的使用者，来仔细考虑添加元素、移除元素以及创建和销毁组件的后果。

  Let's see these dynamics at work. For fun, we'll stack the deck *against*
  our recommendation and consider a component called `heavy-loader` that
  ***pretends*** to load a ton of data when initialized.

  让我们在实践中看看这些变化。为了娱乐，我们设想在甲板上有个叫`heavy-loader`(重型起重机)的组件，它会***假装***在初始化时装载一吨数据。

  We'll display two instances of the component.  We toggle the visibility of the first one with CSS.
  We toggle the second into and out of the DOM with `ngIf`.

  我们将显示该组件的两个实例。我们使用CSS切换第一个实例的可见性，用`ngIf`把第二个实例添加到DOM和将其移除。

+makeTabs(
    `structural-directives/ts/app/structural-directives.component.html,
    structural-directives/ts/app/heavy-loader.component.ts`,
    'message-log,',
    'template (excerpt), heavy-loader.component.ts')

:marked
  We also log when a component is created or destroyed
  using the built-in `ngOnInit` and `ngOnDestroy` [lifecycle hooks](lifecycle-hooks.html).
  Here it is in action:

  借助内建的`ngOnInit`和`ngOnDestroy`[生命周期钩子](lifecycle-hooks.html)，我们同时记录了组件的创建或销毁过程。
  下面是它的操作演示：

figure.image-display
  img(src='/resources/images/devguide/structural-directives/heavy-loader-toggle.gif' alt="heavy loader toggle")

:marked
  Both components are in the DOM at the start.
  First we toggle the component's visibility repeatedly. The component never leaves the DOM.
  When visible it's always the same instance and the log is quiet.

  开始的时候，两个组件都在DOM中。
  首先我们重复切换第一个组件的可见性。组件从未离开过DOM节点。
  当可见时，它总是同一个实例，而日志里什么都没有。

  Then we toggle the second component with `ngIf`.
  We create a new instance every time and the log shows that we're paying
  a heavy price to create and destroy it.

  当我们切换使用`ngIf`的第二个实例时。
  我们每次都会创建新的实例，而日志中显示，我们为了创建和销毁它付出了沉重的代价。

  If we really expected to "wink" the component like this, toggling visibility would be the better choice.
  In most UIs, when we "close" a component we're unlikely see it again for a long time, if ever.
  The `ngIf` would be preferred in that case.

  如果我们真的期望像这样让组件“眨眼”，切换可见性就会是更好的选择。
  在大多数UI中，当我们“关闭”一个组件时，在相当长时间内都不大可能想再见到它 —— 可能永远也不见。
  在这种情况下，我们会更喜欢`ngIf`。

<a id="template"></a>
.l-main-section
:marked
  ## The *&lt;template>* tag
  ## *&lt;template>*标签

  Structural directives, like `ngIf`, do their magic by using the
  [HTML 5 template tag](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template).

  结构型指令，比如`ngIf`，使用[HTML 5的template标签](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/template)
  完成它们的“魔法”。

  Outside of an Angular app, the `<template>` tag's default CSS `display` property is `none`.
  It's contents are ***invisible*** within
  a hidden [document fragment](https://developer.mozilla.org/en/docs/Web/API/DocumentFragment).

  在Angular应用之外，`<template>`标签的默认CSS属性`display`是`none`。
  它的内容存在于一个***隐藏的***[文档片段](https://developer.mozilla.org/en/docs/Web/API/DocumentFragment)中。

  Inside of an app, Angular ***removes*** the`<template>` tags and their children.
  The contents are gone &mdash; but not forgotten as we'll see soon.

  而在Angular应用中，Angular会***移除***`<template>`标签及其子元素。
  这些内容不见了，但是并没有被“忘记”，我们很快就明白了。

  We can confirm these effects by wrapping the middle "hip" of the phrase "Hip! Hip! Hooray!" within a `<template>` tag.

  我们可以通过把短语"Hip! Hip! Hooray!"中间的"hip"包在一个`<template>`标签中来验证下这个效果。

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'template-tag')(format=".")
:marked
  The display is a 'Hip! Hooray!', short of perfect enthusiasm. The DOM effects are different when Angular is in control.

  这时候显示的内容是'Hip! Hooray!'，缺乏完美的热情(译注：因为少了一个词嘛)。在Angular的控制下，DOM的效果是不同的。

figure.image-display
  img(src='/resources/images/devguide/structural-directives/template-in-out-of-a2.png' alt="template outside angular")

:marked
  Evidently Angular replaces the `<template>` tag and its contents with empty `<script>` tags.
  That's just its default behavior.
  It can do something different as we saw when applying a variety of `ngSwitch` directives to `<template>` tags:

  显然，Angular把`<template>`标签及其内容替换成了一个空白的`<script>`标签。
  这只是它的默认行为。
  当把`ngSwitch`家族的各种指令应用于`<template>`标签时，我们就会看到有些东西不一样了：

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngSwitch')(format=".")
:marked
  When one of those `ngSwitch` conditions is true, Angular inserts the template's content into the DOM.

  当这些`ngSwitch`的条件之一为真的时候，Angular把模板的内容插入到了DOM中。

  What does this have to do with `ngIf` and `ngFor`?  We didn't use a `<template>` tag with those directives.

  这和`ngIf`和`ngFor`有什么关系？很明显，我们在那些指令中并没有用到`<template>`标签。

<a id="asterisk"></a>
.l-main-section
:marked
  ## The asterisk (\*) effect
  ## 星号(\*)效果
  Here are those directives again. See the difference?

  下面也是那些指令。看出有什么不同了吗？

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'asterisk')(format=".")
:marked
  We're prefixing these directive names with an asterisk (\*).

  我们把那些指令名加上了星号(\*)前缀。

  The asterisk is "syntactic sugar". It simplifies `ngIf` and `ngFor` for both the writer and the reader.
  Under the hood, Angular replaces the asterisk version with a more verbose `<template>` form.

  这个星号是一种“语法糖”。它简化了`ngIf`和`ngFor` —— 无论是写还是读。

  The next two `ngIf` examples are effectively the same and we may write in either style:

  接下来这两个`ngIf`范例的效果完全相同，只是我们写成了另一种风格：

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngIf-template')(format=".")

:marked
  Most of us would rather write in style (A).

  大多数都喜欢用风格(A)来写。

  It's worth knowing that Angular expands style (A) into style (B).
  It moves the paragraph and its contents inside a `<template>` tag.
  It moves the directive up to the `<template>` tag where it becomes a property binding,
  surrounded in square brackets. The boolean value of the host component's `condition` property
  determines whether the templated content is displayed or not.

  要知道，Angular会把风格(A)写成风格(B)。
  它把段落及其内容移到了`<template>`标签中。
  它把指令移到了`<template>`标签上，成为该标签的一个属性绑定 —— 包装在方括号中。
  宿主组件的`condition`属性的布尔值决定该模板的内容是否应该被显示。

  Angular transforms `*ngFor` in a similar manner:

  Angular把`*ngFor`转换成一个类似的形式：

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'ngFor-template')(format=".")
:marked
  The basic pattern is the same:&nbsp; create a `<template>`, relocate the content,
  and move the directive onto the `<template>`.

  基本的转换模式是一样的：创建一个`<template>`，将内容重定位，并且把指令移到`<template>`上。

  There are extra nuances stemming from
  Angular's [ngFor micro-syntax](template-syntax.html#ngForMicrosyntax) which expands
  into an additional `ngForOf` property binding (the iterable) and
  the `hero` template input variable (the current item in each iteration).

  Angular的[ngFor微语法](template-syntax.html#ngForMicrosyntax)里面有一些细微差别，
  它被展开成了额外的`ngForOf`属性绑定(可迭代者)和一个模板输入变量`hero`(每次迭代中的当前条目)。

<a id="unless"></a>
.l-main-section
:marked
  ## Make a structural directive
  ## 制作一个结构型指令
  Let's write our own structural directive, an `Unless` directive, the not-so-evil twin of `ngIf`.

  我们来写自己的结构型指令：`Unless`，这是`ngIf`指令不那么邪恶的孪生兄弟。

  Unlike `ngIf` which displays the template content when `true`,
  our directive displays the content when the condition is ***false***.

  当条件为`true`时`ngIf`才显示模板内容，与之不同的是，我们这个指令只有当条件是***false***时才显示这些内容。

block unless-intro
  :marked
    Creating a directive is similar to creating a component.

    创建指令很像创建组件。

    * import the `Directive` decorator.

    * 导入`Directive`装饰器。

    * add a CSS **attribute selector** (in brackets) that identifies our directive.

    * 添加一个CSS**属性选择器**(括号中)，来标记出我们的指令。

    * specify the name of the public `input` property for binding
    (typically the name of the directive itself).

    * 指定`input`属性用于绑定的公开名称(通常就是指令自己的名字)。

    * apply the decorator to our implementation class.

    * 把这个装饰器应用到我们的实现类上。

    Here is how we begin:

    下面是最初的样子：

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-declaration', 'unless.directive.ts (excerpt)')(format=".")
.l-sub-section
  :marked
    ### Selector brackets [&nbsp;]
    ### 选择器中的括号[&nbsp;]
    The CSS syntax for selecting an attribute is a name in square brackets.
    We surround our directive name in square brackets. See *Directive configuration* on the
    [cheatsheet](cheatsheet.html).

    在CSS中，用于选择属性(Attribute)的选择器就是放在方括号中的名字。
    于是我们把指令名包裹在方括号中。参见[小抄](cheatsheet.html)中的*指令配置项*。

    ### Selector name prefixes
    ### 选择器名称前缀

    We recommend picking a selector name with a prefix to ensure
    that it cannot conflict with any standard HTML attribute, now or in the future.

    我们建议在给选择器起名时加个前缀，以确保它不会和任何标准的HTML属性冲突，无论是现在还是未来。

    We do **not** prefix our `unless` directive name with **`ng`**.
    That prefix belongs to Angular and
    we don't want to confuse our directives with their directives.

    我们**并没有**给`unless`指令名加上**`ng`**前缀。
    那个前缀是属于Angular的，我们肯定不会希望自己的指令和Angular内建的指令冲突。

    Our prefix is `my`.

    我们用的前缀是`my`。
:marked
  We'll need access to the template *and* something that can render its contents.
  We access the template with a `TemplateRef`.  The renderer is a `ViewContainerRef`.
  We inject both into our constructor as private variables.

  我们需要访问模板，并且*还*需要一个渲染器来渲染它的内容。
  我们通过`TemplateRef`来访问模板。渲染器是`ViewContainerRef`。
  我们把它们都作为私有变量注入到构造函数中。

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-constructor')(format=".")

:marked
  The consumer of our directive will bind a boolean value to our directive's `myUnless` input property.
  The directive adds or removes the template based on that value.

  这个指令的使用者将把一个布尔值绑定到指令的输入属性`myUnless`上。
  该指令会基于这个值添加或移除此模板。

  Let's add the `myUnless` property now as a setter-only property.

  我们现在先把`myUnless`属性定义成一个“只写”属性。

+makeExample('structural-directives/ts/app/unless.directive.ts', 'unless-set')(format=".")
.l-sub-section
  :marked
    The `@Input()` annotation marks this property as an input for the directive.

    `@Input()`装饰器表明这个属性对于指令来说是个输入属性。

:marked
  Nothing fancy here: if the condition is false,
  we render the template, otherwise we clear the element content.

  这里没什么特别的：如果条件为假，我们就渲染模板，否则就清空元素内容。

  The end result should look like this:

  最终看起来是这样的：

+makeExample('structural-directives/ts/app/unless.directive.ts', null, 'unless.directive.ts')

:marked
  Now we add it to the `declarations` array of the AppModule and try it.
  First we add some test HTML to the template:

  现在，我们就来把它加到宿主组件的`directives`数组中，试一试。
  我们首先把一些测试用的HTML添加到模板中：

+makeExample('structural-directives/ts/app/structural-directives.component.html', 'myUnless')(format=".")
:marked
  We run it and it behaves as expected, doing the opposite of `ngIf`.
  When `condition` is `true`, the top paragraph is removed (replaced by `<script>` tags) and the bottom paragraph appears.

  我们运行它，它的行为正如所预期的那样 —— 跟`ngIf`相反。
  当`condition`为`true`时，顶部的段落被移除了(被替换为`<script>`标签)，并且底部的段落显示了出来。
figure.image-display
  img(src='/resources/images/devguide/structural-directives/myUnless-is-true.png' alt="myUnless is true" )

:marked
  Our `myUnless` directive is dead simple. Surely we left something out.
  Surely `ngIf` is more complex?

  这个`myUnless`指令实在太简单了，我们肯定忘了点什么。
  那么`ngIf`会更复杂吗？

  [Look at the source code](https://github.com/angular/angular/blob/master/modules/%40angular/common/src/directives/ng_if.ts).
  It's well documented and we shouldn't be shy
  about consulting the source when we want to know how something works.

  [看下源码](https://github.com/angular/angular/blob/master/modules/%40angular/common/src/directives/ng_if.ts)。
  它有很好的文档，况且，如果我们想了解某些东西的工作原理，也不用羞于“咨询”源码。

  `ngIf` isn't much different! There are a few
  additional checks to improve performance (don't clear or recreate the
  view unless necessary) but otherwise it's much the same.

  `ngIf`也没多大不同嘛！它做了一些额外的检查来提升性能(除非必要，否则它不会清除或重新创建视图)，但其它的部分都跟我们写的一样。

.l-main-section
:marked
  ## Wrap up
  ## 总结
  Here is the pertinent source for this chapter.

  本章相关的代码如下：

+makeTabs(`
  structural-directives/ts/app/unless.directive.ts,
  structural-directives/ts/app/heavy-loader.component.ts,
  structural-directives/ts/app/structural-directives.component.ts,
  structural-directives/ts/app/structural-directives.component.html
  `,
  null,
  `unless.directive.ts,
   heavy-loader.component.ts,
   structural-directives.component.ts,
   structural-directives.component.html
   `)
:marked
  We learned that we can manipulate our HTML layout with
  structural directives like `ngFor` and `ngIf` and we
  wrote our own structural directive, `myUnless`, to do something similar.

  我们学会了通过像`ngFor`和`ngIf`这样的结构型指令来操纵HTML的布局。我们还写出了我们的第一个结构型指令`myUnless`来做类似的事情。

  Angular offers more sophisticated techniques for managing layout
  such as *structural components* that can take external content
  and incorporate that content within their own templates.
  Tab and tab pane controls are good examples.

  Angular提供了更多成熟的技术来管理布局，比如*结构性组件*可以接受外部内容，并把这些内容合并到组件自己的模板中。
  多页标签及其面板控件就是很好的例子。

  We'll learn about structural components in a future chapter.

  我们将在未来的章节中还会讲述结构型指令。
